The present invention relates to variable speed inverter drive circuits and more particularly to inverter drive circuits integrated with a boost converter.
Increasingly, variable speed inverter drive circuits are used in the automotive environment. A number of electrical payloads, which a car battery must feed is increasing with every new car model that is put on the market. It is understood that demand for increased battery power will, in the medium term, oblige most car manufacturers to move from a present 12V battery architecture to higher battery voltages, e.g., 42V, or to a dual battery architecture.
In the meanwhile, the 12V battery must satisfy this increasing demand of electrical loads. Within these electrical loads, variable speed motors play a major role, including, as an example, car air conditioning compressor and fan motors, and, in high end vehicles, the compressor for an on-board refrigerator that may be used, for example, to cool down beverages. Another example of an electrical load is a big electrical motor driven compressor, e.g., in a refrigerated truck that is used to cool down a cabin in which delicate foods are transported. Such motors are typically brushless permanent magnet type motors with trapezoidal excitation.
Since the power required by these motors often exceed a few hundreds of Watts, the motors cannot be designed to be directly fed by a battery voltage, i.e., through an inverter. That is because an otherwise excessive current would be drawn, and copper losses in the wiring and circuit components would be unacceptable. For that reason, as shown in FIG. 1, a boost converter 10 having a boost inductor 14, a diode 18, a switch 16, and a bulk capacitor 22 may be interposed between the battery 12 and the inverter 20 having half bridge stages U, V, W for feeding three phases of a motor 24. The motor 24 itself may run at 50-100V, which creates a much lower current stress on both the motor and the inverter switches.
Such boost converter is quite critical. Because its input voltage is low, cold battery's voltage can be as low as 8V, an input current is very high. This imposes the use of bulky and expensive semiconductors and passive components. For example, a 250 W motor would require a boost circuit able to handle an input current in excess of 30 A on average, and, perhaps, 35 A peak, depending on the ripple allowed on the input inductor which, in turn, affects the size and the power dissipation by magnetic losses of the inductor itself.